1. Field of the Invention
The present invention relates to an optical low-pass filter which is disposed in front of a solid-state image sensor, and more particularly to an optical low-pass filter of phase grating type whereby excellent image output is obtained by suppressing the high spacial frequency component of an object producing a signal by virtue of this solid-state image sensor.
2. Related Background Art
It is well known that a color solid-state image sensor having a predetermined sampling pitch produces a spurious signal for an object having a high spacial frequency above a predetermined level. To suppress this spurious signal, it is common practice to provide an optical low-pass filter composed of crystal plate or an optical low-pass filter of phase grating type in front of the solid-state image sensor.
The crystal optical filter has a problem in that, although it can provide relatively ideal low-pass characteristics, the fabrication is difficult and the cost is high.
On the other hand, the optical low-pass filter of phase grating type has an advantage in that it can be fabricated at relatively low cost, although it is inferior with respect to the optical characteristics. It has been used in several video cameras, as proposed in, for example, Japanese Patent Application Laid-open No. 63-311323, Japanese Patent Publication No. 52-22247 and Japanese Patent Publication No. 56-2936.
By the way, the optical low-pass filter of phase grating type has a property that the low-pass effect abruptly decreases in a small aperture state, particularly if the aperture reaches within two periods of phase grating, as pointed out in Japanese Patent Publication No. 52-22247, for example.
This is because the optical low-pass filter of phase grating type obtains the low-pass effect by causing diffraction with a plurality of gratings, and the desired diffraction effect is reduced in such a small aperture state.
Therefore, in a video camera using the optical low-pass filter of phase grating type, a plurality of ND filters have been affixed to a portion of the aperture, or the density of ND filter has been increased, for example, so that the least aperture state may not be less than two periods, thereby bringing about high cost and the decreased quality of unfocused state.